The content of metal oxides and/or their precursors present in base ore, refined ore, incinerator ash and the like is at most several percent by mass. For example, one known process for recovering vanadium oxide from base ore containing such a small amount of vanadium oxide involves roasting the vanadium oxide-containing ore for several hours together with soda ash (sodium carbonate) in an oxidizing atmosphere at an elevated temperature of 900 to 1200° C. to form sodium vanadate, extracting the sodium vanadate with water to obtain an aqueous solution of sodium vanadate, then additionally refining to ultimately yield vanadium oxide (U.S. Pat. No. 3,320,024).
However, in this method of recovery, heating the several percent by weight or less of vanadium oxide within the ore for a long period of time at an elevated temperature of about 1000° C. requires a large amount of energy. Additional problems include the large scale of the equipment needed, high investment costs, and the strict specifications for high-temperature durability of the facility. Moreover, because this process involves a high-temperature reaction with an alkali, the bricks making up the equipment undergo alkali corrosion, so that maintenance is difficult (JP 2001-519751 A).
Likewise, in the preparation of a complex oxide by roasting chromium ore containing several tens of percent by weight of chromium oxide together with an alkali metal compound, because it is necessary to heat the alkali metal compound to 1000 to 1200° C., problems like those in the case of vanadium oxide similarly exist (JP 48-38817 A).
A process has been proposed for reacting tungsten oxide-containing wolframite or scheelite with an alkali metal compound at 500 to 800° C. to obtain water-soluble tungsten valuables which are then extracted with water. However, this process has a low yield, in addition to which it is necessary to heat and re-react the extraction residue (JP 55-89446 A)
Mechanochemical treatment, which refers to reaction processes that do not involve heating, are also known. Such treatment is typically carried out by applying mechanical energy to a solid substance such as by shearing, compression, impact, grinding, bending, stretching, then influencing the chemical state such as by bringing about chemical changes in gaseous and liquid substances located near the solid substance, or by directly inducing, or promoting, chemical changes between these gaseous and liquid substances and the surface of the solid substance. Various treatment methods of this type have been described.
For example, JP 11-71111 A describes a method for the extraction of rare-earth metal-containing substances in which a rare earth metal-containing substance is mechanochemically treated in a planetary mill, then leached with a low-concentration acid.
JP 2001-11549 A discloses an indium-containing compound leaching process which involves subjecting an indium-containing compound to mechanochemical treatment in the presence of a ceramic powder, then leaching the compound with a low-concentration acid at ambient temperature.
In addition, JP 11-310442 discloses a process in which calcium oxide is mixed with coal ash and mechanochemically treated, thereby producing a hydraulic treated product in powder form.
However, because mechanochemical treatment is based on the promotion of reactions by point contact at active sites on solid substance surfaces renewed by mechanical action such as impact, increasing the amount of reaction that takes place (i.e., the amount of product) requires that a means be employed for continuing such treatment over a long period of time so as to repeatedly renew the surface. Hence, this approach is fundamentally unsuitable for production on an industrial scale.